Conventional medical images may be generated via transmission imaging or emission imaging. According to transmission imaging, the imaging source (e.g., an X-ray source) is external to the subject and the source radiation (e.g., X-rays) is transmitted through the subject to a detector. According to emission imaging, the imaging source (e.g., a gamma ray-emitting radiopharmaceutical) is internal to the subject (e.g., due to injection or ingestion thereof) and the source radiation (e.g., gamma rays) is emitted from within the subject to a detector. In either case, absorption or scattering within the subject tissue attenuates the source radiation prior to reception of the source radiation by the detector.
Images are created by determining the attenuation caused by the subject tissue. This determination is relatively straightforward in the case of transmission imaging, since the amount of the external source radiation being transmitted through the subject and the amount received at the detector are both known. However, the determination of attenuation in emission imaging is more difficult, because the amount of radiation being emitted by the emission source(s) within the subject cannot be measured directly.
Accordingly, in emission imaging such as single-photon-emission-computer-tomography (SPECT) and positron-emission-tomography (PET), image reconstructions incorporate attenuation corrections to generate visually realistic and clinically accurate images. The most common way attenuation corrections are based on Linear Attenuation Coefficient (LAC) maps (“mu-maps”) derived from a Computed Tomography (CT) scan of the subject tissue. Such a CT scan is typically performed during the same imaging session at which the emission imaging is performed.
FIG. 1 illustrates conventional attenuation-corrected reconstruction. A set of two-dimensional emission images (i.e., emission data) is acquired and a CT scan is performed substantially contemporaneously to acquire CT data. For example, emission data of a portion of a patient may be acquired while the patient is positioned in an imaging position, and CT data of a similar portion of the patient may be acquired via a CT scan while the patient remains substantially in the imaging position. A transform is applied to the CT data to generate a mu-map. Several techniques exist generating a mu-map from CT data. Lastly, a reconstruction process generates an attenuation-corrected volume based on the emission data and the mu-map.
The CT scan required to generate the mu-map undesirably delivers a corresponding radiation dose to the subject tissue. Also, some emission imaging scenarios lack the availability of a contemporaneous CT scan. What is needed are efficient systems to reconstruct emission data with attenuation correction without requiring a CT scan. Attempts to address this need have utilized Radon transform and image segmentation to estimate a mu-map but have not received satisfactory results in terms of quality of the resulting reconstructed image and/or ease of use.